Brake systems are critical to the safe operation and control of a motor vehicle. In conventional vehicle brake systems, the vehicle driver depresses the brake pedal to decelerate the vehicle and the vehicle brake system responds with a reactive force back to the driver through the pedal that the driver may mentally relate to the amount of braking torque applied by the vehicle braking system. In a hydraulic boosted or assisted braking system, as the driver depresses the brake pedal the booster device generates a brake line pressure corresponding to a multiple (also known as boost ratio) of the driver applied pedal pressure. In this way, the driver's applied brake pedal force is multiplied to provide greater friction braking at the wheels of the vehicle. Similar to the earlier non-boosted brake system discussion, the boosted brake system typically responds with a reactive force through the brake pedal back to the driver by which the driver can gauge the driver's requested braking torque (or requested vehicle deceleration).
In a hybrid electric vehicle or pure electric vehicle the brake systems are more complicated than the brake systems described above. Hybrid electric vehicles are characterized in that they utilize one or more electric motors or motor/generators to convert stored electrical energy from a battery into the kinetic energy of the moving vehicle. In such vehicles, stored electrical energy in the battery is a limited and precious resource. Therefore, hybrid electric vehicles typically employ one or more generators or motor/generator devices adapted to provide a braking torque by converting or regenerating a portion of the kinetic energy of a moving vehicle into electrical energy stored back to the vehicle's batteries. Such braking is known as regenerative braking and is preferred to the fullest extent possible over friction braking as the kinetic energy is recovered rather than being dissipated as heat through friction brakes.
In mechanical brake systems such as the non-hybrid brake systems discussed above, a failure or degradation in the operation of the brake system due to a fault such as a booster pump fault is immediately noticeable by the driver as the brake system typically requires more brake pedal force from the driver to stop the vehicle.